In recent years, variable resistance nonvolatile memory devices (hereinafter, also simply referred to as “nonvolatile memory devices”) having memory cells including variable resistance nonvolatile memory elements (hereinafter, also simply referred to as “variable resistance elements”) have been researched and developed. The variable resistance elements are elements that have characteristics in which a resistance value reversibly changes based on electrical signals and are capable of holding data corresponding to the resistance value in a nonvolatile manner.
Commonly known as a nonvolatile memory device including variable resistance elements is a nonvolatile memory device including a matrix of so-called 1T1R memory cells in each of which a MOS transistor and a variable resistance element are connected in series with each other at a location close to a cross-point between a bit line and a word line that are arranged perpendicular to each other. In each of the 1T1R memory cells, the variable resistance element has one of terminals connected to the bit line or a source line, and the other terminal connected to a drain or source of the MOS transistor. The transistor has a gate connected to the word line. The transistor has the other terminal connected to the source line or bit line that is not connected to the other terminal of the variable resistance element. The source line is arranged parallel to the bit line or the word line.
Also generally known as another memory cell structure is a nonvolatile memory device including a matrix of cross point memory cells called 1D1R memory cells in each of which a diode and a variable resistance element are connected in series with each other at a cross-point between a bit line and a word line that are arranged perpendicular to each other.
The following describes typical conventional variable resistance elements.
Non Patent Literature (NPL) 1 discloses a nonvolatile memory including 1T1R memory cells each using a transition metal oxide as a variable resistance to element. NPL 1 recites that in order to allow a resistance value of a transition metal oxide thin film, generally an insulator, to be changed by application of an electrical pulse, it is necessary to perform forming processing to form a conductive path for switching the variable resistance element between a high resistance state and a low resistance state.
FIG. 13 is a graph showing a dependency of a forming voltage (V_form) on a transition metal oxide film thickness (TMO Thickness) that is disclosed in NPL 1. The graph shows four property types of NiO, TiO7, HfO2, and ZrO2 as transition metal oxides. A necessary forming voltage depends on the types of the transition metal oxides, and increases as a transition metal oxide has a greater film thickness. As a result, in order to reduce a forming voltage, a transition metal oxide such as MO is selected and the film thickness of the transition metal oxide is decreased, for example.
Patent Literature (PTL) 1 discloses a nonvolatile memory including ion conductive variable resistance elements each of which includes an insulator film (amorphous Gd2O3) and a conductive film (CuTe).
FIG. 14 is a schematic cross-section view of a variable resistance element disclosed in PTL1.
A variable resistance element 5 has a stack structure in which a conductive film 3 and an insulator film 4 are disposed between electrodes 1 and 2. PTL 1 discloses that examples of a material of the conductive film 3 include a metal film including at least one metal element selected from among Cu, Ag, and Zn, an alloy film (e.g., CuTe alloy film), and a metal compound film, and that examples of a material of the insulator film 4 include amorphous Gd2O3 and an insulator such as SiO2.
In regard to programming the variable resistance element 5, when a voltage causing an electric potential of the first electrode 1 to be lower than that of the second electrode 2 is applied, ions of the metal enter the insulator film 4 by being pulled toward the electrode 2. When the ions reaches the electrode 2, the electrodes 1 and 2 are conducted, and the variable resistance element 5 changes to a low resistance state (LR writing). In this way, writing data into the variable resistance element 5 (LR writing) is performed. Conversely, when a voltage causing the electric potential of the electrode 1 to be higher than that of the electrode 2, the metal element is ionized, and ions of the metal element come out of the insulator film 4 by being pulled toward the electrode 1. As a result, insulating properties between the electrodes 1 and 2 increase, and the variable resistance element 5 changes to a high resistance state (HR writing). In this way, erasing data from the variable resistance element 5 (HR writing) is performed.
Each of (a) and (b) in FIG. 15 is a wave form chart of a voltage pulse applied to the variable resistance element 5 when data recording is performed once.
(a) in FIG. 15 shows a pulse wave form when writing (recording of data indicating “1”) is performed. An erase voltage pulse PE is first applied as a voltage pulse having a reverse polarity, and a voltage pulse PW having a polarity corresponding to the data to be recorded is subsequently applied. In other words, a voltage pulse P1 for recording the data indicating “1” includes a set of the voltage pulses PE and PW.
(b) in FIG. 15 shows a pulse wave form when erasing (recording of data indicating “0”) is performed. A write voltage pulse PW is first applied as a voltage pulse having a reverse polarity, and a voltage pulse PE having a polarity corresponding to the data to be recorded is subsequently applied. In other words, a voltage pulse P0 for recording the data indicating “0” includes a set of the voltage pulses PW and PE.
Recording data into the variable resistance element 5 by using the voltage pulses P1 and P0 shown in (a) and (b) in FIG. 15 limits the number of times the voltage pulse PW or PE having the same polarity succeeds one after another, to two or less. With this, it is possible to control a change of a resistance value of the variable resistance element 5 (an increase of a resistance value from LR state or a decrease of a resistance value from HR state) which is caused by successive application of the voltage pulse PW or PE having the same polarity many times, thereby extending an operating life.